Corrosion-resistant aluminum-copper-magnesium-zinc powder metallurgy alloys

ABSTRACT

ALUMINUM BASE POWDER METALLURGY ALLOY ARTICLE HAVING AN IMPROVED COMBINATION OF HIGH TRANSVERSE YIELD STRENGTH AND STRESS CORROSION CRACKING RESISTANCE. THE ALLOY CONTAINS THE BASIC PRECIPITATION HARDENING ELEMENTS ZINC, MANGSIUM AND COPPER. IT MAY ADDITIONALLY CONTAIN COBALT OR MANGANESE. THE ALLOY IS PREPARED BY ATOMIZATION OF A MELT OF THE ELEMENTS HOT WORKING, SOLUTION HEAT TREATING, QUENCHING AND TWO-STAGE ARTIFICIAL AGING. COMPONENTS OF THE ALLOY IN PERCENT BY WEIGHT ARE, IN ADDITION TO THE ALUMINUM, 5 TO 13 ZINC, 1.75 TO 6 MANGNESIUM, 0 TO 2.5 COPPER, AND UP TO ABOUT TO COBALT OR MANGANESE. UP TO 0.75 BY WEIGHT CHROMIUM AND UP TO 0.25 BY WEIGHT ZIRCONIUM MAY BE PRESENT WHEN COBALT OR MANGANESE IS ALSO PRESENT.

United States Patent 3,563,814 CORROSION-RESISTANT ALUMINUlVl-COPPER- MAGNESIUM-ZINC POWDER METALLURGY ALLOYS John I. Lyle, Jr., New Kensington, Pa., Raymond J. Towner, Lima, Ohio, and Allan P. Haarr, Delmont, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 8, 1968, Ser. No. 719,753 Int. Cl. C22c 21/00; C22f 1/04 U.S. Cl. 14812.7 5 Claims ABSTRACT OF THE DISCLOSURE Aluminum base powder metallurgy alloy article having an improved combination of high transverse yield strength and stress corrosion cracking resistance. The alloy contains the basic precipitation hardening elements zinc, magnesium and copper. It may additionally contain cobalt or manganese. The alloy is prepared by atomization of a melt of the elements, hot working, solution heat treating, quenching and two-stage artificial aging. Components of the alloy in percent by weight are, in addition to the aluminum, 5 to 13 zinc, 1.75 to 6 magnesium, 0 to 2.5 copper, and up to about 3 cobalt or manganese. Up to 0.75 by weight chromium and up to 0.25 by weight zirconium may be present when cobalt or manganese is also present.

BACKGROUND OF THE INVENTION This invention relates to aluminum-copper-magnesiumzinc alloys prepared by powder metallurgy techniques. More particularly, it pertains to improving tensile and stress corrosion properties of articles prepared from aluminum-copper-magnesium-zinc alloys by the addition of certain dispersion strengthening elements to the melt from which the alloys are prepared by atomization. By powder metallurgy techniques we mean those processes in which the molten alloy is atomized to make fine powder, the powders are compacted, and the compact is fabricated into the desired form by hot working.

The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

One way in which to improve tensile properties, that is, in general, to strengthen, aluminum base alloys is by recipitation hardening. This occurs when a supersaturated solid solution precipitates its excess solute. The process is favored in alloy systems with appreciably greater solubility for the solute at elevated temperatures than at lower or ambient temperatures. The precipitate effects strengthening of the structure by setting up coherency strains in the matrix by the precipitated particles. Zinc, magnesium and copper are well known as elements which contribute substantially to precipitation hardening. It is also known that one way of improving strengthening is to add to the alloy system one or more elements which form compounds having low solubilities and low diffusibilities in the solid state at elevated temperatures to cause a strengthening effect which lasts even after extensive heat treatment. This type of reaction is commonly referred to as dispersion strengthening or interference hardening.

Copper, magnesium and zinc have been disclosed to be useful precipitation hardening elements for use in aluminum base alloys. Individual dispersion strengthening elements which have been suggested for use in improving the properties of aluminum base alloys include manganese, iron, nickel, chromium, titanium, vanadium, zirconium, cobalt, molybdenum and tungsten. Thus far it has not proved too difiicult to add precipitation hardening elements because of their characteristic high liquid solubilities and high solid solubilities and high solid diffusiblities at high temperatures. However. for addition of dispersion strengthening elements standard ingot casting procedures have been of limited use when it was desired to modify the alloy composition by addition of considerable amounts of these elements. This has been due to the fact that dispersion strengthening elements are characterized by low liquid solubilities near the solidification temperatures and low solid solubilities and low solid diffusibilities at elevated temperatures.

It has been proposed to combine the strengthening characteristics of both precipitation hardening and dispersion strengthening elements by using atomization of alloys from the melt to permit use of higher concentrations of alloy elements than is possible in ingot metallurgy where the casting solidification rates are slow relative to atomizing solidification rates. For example, use of atomized alloy powders has helped improve the strength properties of aluminum base alloys by resulting in a structure which I is several orders of magnitude finer than standard ingot metallurgy alloy structures.

While considerable improvement in strength has been possible by the aforementioned combination of dispersion strengthening and precipitation hardening, particularly with the improved results obtained by atomization of a melt of the alloy containing the dispersion strengthening elements, improvements in stress corrosion cracking resistance, one of the main features desired for lightweight rocket motor and aircraft parts which are required to withstand high stress conditions, have thus far been rather limited, probably partly because of the fact that, in increasing stress corrosion resistance by standard age hardening procedures, the strength properties of the alloys have been somewhat impaired, making them not entirely satisfactory for some applications where conditions conducive to stress-corrosion cracking might be expected.

OUTLINE OF THE INVENTION Accordingly, it is an object of this invention to provide a novel hot Worked powder metallurgy alloy article which combines a high transverse yield strength with a high stress corrosion cracking resistance. Another object is to provide a method for production of novel powder metallurgy alloys which have a transverse yield strength comparable to that of known aluminum-copper-magnesium-zinc ingot metallurgy alloys plus a higher resistance to stress corrosion cracking after two-stage artificial aging. These and other objects of the invention will be apparent from the description and claims which follow.

By high transverse yield strength we mean one which is equal to or superior to that of presently known aluminum base ingot metallurgy alloy articles containing the precipitation hardening elements copper, magnesium and zinc. Such presently known aluminum base ingot metallurgy alloy articles have a transverse yield strength of around 60,000 to 68,000 p.s.i. By high stress erosion cracking resistance we mean one which is greater than that of the aforementioned ingot metallurgy alloy articles containing copper, magnesium and zinc. Such alloy articles containing copper, magnesium and zinc fail, that is, crack or break apart, in less than 84 days when subjected to stress in a transverse direction of 50% of their transverse yield strength in the standard test of alternate immersion in a 3.5% by weight sodium chloride solution [ASTM STP425 (1967), pp. 8 and 188].

In accordance *with this invention a hot-worked powder metallurgy alloy article is prepared in which the alloy contains in percent by weight 5-13 zinc, 1.75-6 magnesium and -2.5 copper. The alloy may additionally contain up to 3 cobalt or manganese, preferably 0.25 to 2, and, when the cobalt or manganese is present, up to about 0.75 chromium and up to about 0.25 zirconium. The alloy may also contain a total of up to about 1.5 A1 0 and up to about 1 percent by weight of other elements, such as titanium, silicone, vanadium, or the like.

One feature of the alloy article of this invention is the aforementioned combination of high resistance to stress corrosion cracking and high transverse yield strength. The powder metallurgy alloy article of the present invention solution heat treated and then aged in two stages does not fail when subjected for a least 84 days under stress in a transverse direction of 50% of its transverse yield strength to the above-mentioned standard alternate immersion in 3.5 NaCl. It is this ability to withstand 84 days in such a corrosive environment at 50% stress which is one factor in making the difference between an alloy article which is commercially acceptable and one which is not for withstanding the highly demanding environments necessarily encountered by modern structural airframe parts.

To obtain this combination of high stress corrosion resistance coupled with high transverse yield strength, the alloy articles of this invention are prepared by atomization of an alloy melt, hot working, solution heat treating and two-stage aging, the first stage at from about 225 F. to about 275 F. for from about 6 to about 96 hours and the second at from about 315 F. to about 350 F. for from about 3 to about 40 hours. The solution heat treatment is preferably at a temperature of from about 750 to 1000 F. The solution heat treatment and aging procedures described in Sprowls et al. U.S. 3,198,676 for Al-Mg-Zn-Cu ingot metallurgy articles may be used in preparing the powder metallurgy articles of this invention.

The basic steps in the fabrication of the aluminum powder metallurgy alloys of combined high transverse yield strength and high stress corrosion cracking resistance of the present invention are as follows:

(1) melting and alloying,

(2) air atomizing and screening to make 100 mesh powder with a Sharples Micromerograph mass median diameter (MMD) of -60 microns,

(3) compacting to 65-90% density in a tapered or split die to permit ejection of the compact without cracking it,

(4) degassing by heating in a fiowing non-oxidizing atmosphere,

(5) hot compacting to substantially density, and

then

(6) hot working.

The hot working may be done by extruding, hot coining, forging, rolling or the like. Additional hot working may be helpful after hot coining.

The solution heat treatment which helps to achieve the improved stress corrosion cracking resistance properties of the alloy articles of this invention involves heating them as mentioned above and as suggested for contrasted ingot metallurgy alloys containing copper, magnesium, zinc and aluminum in above-mentioned U.S. Pat. 3,198,676 to our co-workers Sprowls et al. to a temperature within the range of about 750 to 1000 F., but below the temperature of incipient fusion, and holding them within that range for a length of time sufficient to obtain substantially complete solution of the zinc and magnesium, and also of the copper when present. Generally this can be accomplished within a period of from 3 or 4 minutes up to about 10 hours, depending on the thickness of the article being treated and whether the surface of the article is directly exposed to the heating medium. At the conclusion of the solution heat treatment the articles may be rapidly cooled to substantially room temperature, for example, by quenching in water at temperatures below about F. Cooling in this manner serves to retain a substantial portion of the dissolved components in a state of solid solution. However, by employing hot water instead of cold water it is possible to further minimize stresses which may be induced by quenching. The above-mentioned two-stage aging treatment then follows.-

At lower temperatures and for shorter periods of time than those suggested above for the first stage of aging the precipitation may be insufficient to provide the proper metallurgical condition for the application of the succeeding aging step in which the alloys are given the final desired high stress corrosion cracking resistance and high transverse yield strength. At higher temperatures and for longer periods of time than those set out above for the first step of aging precipitation may be carried too far, that is, to a point at which it may adversely affect the precipitation produced in the second stage or step and thereby possibly result in low transverse yield strength and failure to obtain the desired combination with high stress corrosion resistance.

The examples in the table which follows are illustrative of the improved powder metallurgy aluminum base alloy articles of the present invention. In each instance the sample survived at least 84 days without stress corrosion cracking failure when subjected to the above-mentioned standard test of alternate immersion in a 3.5% 'NaCl solution under a tension of 50% of its transverse yield strength. Prior art Al-Zn-Mg-Cu ingot metallurgy alloy samples of similar composition but with no elements other than the Al, Mg, Zn and Cu, except for not more than a total of 1% of other elements present as impurities, failed under these conditions in less than 84 days. Each sample in the table was cold water quenched after solution heat treatment at 860 F. for 2 hours. The sec- 0nd stage aging in each instance was at 330 F. The first stage aging was at 250 F. for 24 hours except for sample 12, in which instance it was for 6 hours at 250 F.

The samples tested were prepared by atomizing a melt of the alloy to produce finely divided cast particles and screening so that most of the particles passed through a 100 mesh screen. Each sample of atomized powder was compacted, heated to below its fusion point in an argon atmosphere and then compressed and extruded. Extrusion sizes were about 4.5 to 4.6 feet in length by about 2 inches in diameter.

TABLE I Second Transverse stage yiel Percent by weight aging strength, time p.s.i. Zn Mg Cu Mn Cr Zr Ti (hours) (thousands) s 75.0 3.5 1.5 0.5 is 07.5 as 63.2 4.0 1.0 0.5 17 67.5 2.0 2.1 1.8 0.1 6&5 3.6 2.3 32 g It is evident from the foregoing data that according 20 about 0.75 chromium, up to about 0.25 zirconium and to our invention we have provided a powder metallurgy balance aluminum, hot working the resulting compacted alloy article of combined high stress corrosion cracking powder, solution heat treating the resulting article, aging resistance and high transverse yield strength. One of said article at from about 225 F. to about 275 F. for the advantages of our invention is the opportunity to from about 6 hours to about 96 hours and then further vary the conditions for the required two-stage aging of -35 aging said article at a temperature of from about 315 F. our alloy articles within the established ranges so as to to about 350 F. for from about 3 hours to about optimize the improved properties for the particular alloy hours, thereby imparting to said article a high stress corarticle used. rosion cracking resistance and a high transverse yield Whereas particular embodiments of the invention have strength. been described above for purposes of illustration, it will 30 5. A process for improving the stress corrosion crackbe evident to those skilled in the art that numerous variaing resistance and transverse yield strength of an alumitions of the details may be made without departing from num base powder metallurgy alloy article which comthe invention as defined in the appended claims. prises atomizing an aluminum base alloy consisting es- Having thus described our invention and certain emsentially of in percent by weight 5 to 13 zinc, 1.75 to bodiments thereof, we claim: 6 magnesium, 0 to 2.5 Cu, 0 to 3 Co or Mn, up to about 1. A hot-worked aluminum base alloy powder article 0.75 chromium, up to about 0.25 zirconium and balance formed from atomized powder of aluminum base alloy aluminum, compacting the atomized alloy to from about consisting essentially of in percent by weight 5 to 13 zinc, 65 to about 90% density, then degassing the compacted 1.75 to 6 magnesium, 0 to 2.5 Cu, 0 to 3 Co or Mn, up alloy, hot working the degassed compacted alloy, solution to about 0.75 chromium, up to about 0.25 zirconium and heat treating the resulting article, aging said article at balance aluminum, said hot-worked article characterized fr m about 225 F. to about 275 F. for from about 6 by having been degassed after compacting and when in hours to about 96 hours and then further aging said ara solution heat treated and two-stage aged condition by ticle at a temp of from about 3 t a t a high stress corrosion resistance and a high transverse 350 F. for from about 3 hours to about 30 hours. yield strength.

2. The article of claim 1 wherein the two-stage aged References Cited condition results from first heating at a temperature from NITE STATES PATENTS about 225 F. to about 275 F. for from about 6 to 2809891 10/1957 Ennor et a1 about 96 hours and then heatmg at a temperature of from about 315 F to about 350 F for from about 3 hours 00 3198676 8/1965 Sprowls et 148-325 to about 40 hc'mrs 5,493 8/1966 Foerster 75-142 3,291,654 12/1966 Foerster 148-159 3. The artlcle of cla1m 1 wherein the alloy also con- 3 307 978 3/1967 Foerster 148 12 7 tains up to about 1.5% by weight A1 0 4. A process for preparation of an improved stress cor- L DEW AYNE RUTLEDGE, Primary Examiner rosion cracking resistant aluminum base powder metallurgy alloy article which comprises atomizing, compacting and then degassing an aluminum base alloy consisting essentially of in percent by weight 5 to 13 zinc, 1.75 to 6 magnesium, 0 to 2.5 Cu, 0 to 3 Co or Mn, up to W. W. STALLARD, Assistant Examiner US. Cl. X.R. 

